A direct conversion receiver is a receiver that multiplies a RF signal received in an antenna by a carrier (local signal) of substantial same frequency and thus directly converts the RF signal into a baseband signal without eliminating conversion into a signal of intermediate frequency, and contributes to miniaturization, weight reduction and low power consumption in a radio receiver.
The direction conversion receiver is described in, for example, Japanese Laid-Open Patent Publication No.H10-247953.
However, in the direct conversion receiver, there is a problem that a direct current offset (hereinafter referred to as “DC offset”) specific to the circuitry arises.
As measures against the DC offset, there is a method that a high-pass filter is inserted into a signal path to cut off a DC component using a capacitor, as shown in FIG. 11 in the above-mentioned Japanese Laid-Open Patent Publication No.H10-247953.
The inventor of the present invention considered installing a direct conversion receiver in a CDMA receiver such as a cellular telephone.
As a result, it was made clear that an AGC circuit indispensable to a CDMA receiver causes a DC offset to occur, and that the above-mentioned method of inserting a high-pass filter into a signal path does not solve the problem with the DC offset caused by the AGC circuit.
This problem is explained below.
In the case of a CDMA receiver, in either area of a weak electric field and storing electric field, the receiver always distinguishes information of a terminal provided with the receiver from data of another user accurately on a channel, and therefore, needs to have an AGC circuit that keeps amplitude of a signal to input to an A/D converter within a predetermined range.
The basic operation of the AGC circuit is measuring power of a received signal, comparing the power with a target value to generate a control signal, and varying a gain of a variable gain amplifier using the control signal (negative feedback control operation).
In the CDMA receiver, it is required to adjust at high speed a convergence coefficient of a variable gain amplifier so as to adapt to a current receiving state, particularly, in cases of immediately after the power is supplied, of immediately after the circuitry is activated in intermittent reception (where the cellular telephone in a stand-by state checks synchronization with a base station intermittently, while the circuit power supply is turned off to be in a low power consumption mode in other states), of performing handover between cells with different frequencies in a compressed mode, and of performing handover between base stations with different systems, for example, in Asian areas where base stations with different systems exist such as a W-CDMA system and GSM (Global System for Mobile communications), and in such cases, it is necessary to increase the gain of the negative feedback loop.
In other words, it is required to decrease an interval for updating a gain of a variable gain amplifier, while increasing a variation width of a control value per-one gain update.
When the gain of a variable gain amplifier is thus updated frequently by a large variation width, voltage fluctuations caused by gain switching are conveyed to a capacitor of the above-mentioned high-pass filter existing on a signal path, and as a result, an acute differential waveform is instantaneously output.
The differential waveform converges with elapsed time. However, when another differential waveform is subsequently output before the convergence, the differential waveforms overlap one another as shown in FIG. 1, and as a result, the DC voltage in the circuitry deviates greatly. In other words, a large DC offset occurs.
When such a large DC offset occurs, the accuracy in demodulated signal deteriorates, and accurate AGC becomes difficult.
As described above, a direct conversion receiver needs a high-pass filter to cut off a DC component, while a CDMA receiver needs an AGC circuit, and the high-pass filter is necessarily a structural element of the AGC loop.
Then, for example, at the initial time of AGC such as the time the power is supplied, it is also necessary to increase the convergence coefficient of AGC (increase the gain variation amount) so as to respond to external propagation environments at high speed. Thus, when increasing the response capability of AGC to hasten the convergence, as described above, the DC offset is generated by overlapping of differential waveforms output from a high-pass filter in synchronization with switching of gain of a variable gain amplifier, resulting in contradictory facts that it finally becomes difficult to perform accurate AGC, and that the time increases required for the gain of the gain variable amplifier to converge.
Thus, when a direct conversion receiver is applied to, for example, a CDMA receiver equipped with an AGC circuit, the self-contradiction arises in the AGC operation, and accordingly, it is difficult to actually use the direct conversion receiver as a CDMA receiver.